To embed a YouTube video you need to copy that videos ID. To obtain a videos ID you simply click the share link underneath the video:

In the link circled above copy what comes after the / sign in the url. The above videos ID will be Gmj3B_-egnc

To embed this video in a post you click on the YouTube button circled in the pic below:

Next paste the videos ID which is Gmj3B_-egnc between the YouTube tags as shown in the pic below:

That's all there is to embedding a YouTube video. The most common mistake members make is placing the videos full url between the YouTube tags which won't work. All that needs to be placed between the YouTube tags is what comes after the / sign in the videos share url located under the video.